Simple Question

Hi, I've never had a washing machine before and I'm about to get a new one delivered and I just have one question that no one on the internet seems to be answering....I understand that I must connect the water pipe inlet connection to the tap and then turn the tap to the 'On' position, but when my washing machine is turned off and the washing machine stops water from entering, where does all that water go from the tap that is still in 'On' position? ( I know some people turn it off preventing leaks but I'm asking what stops that flow of water from the tap in 'On' position in the first instance - how does it work? What stops usage of water when the washing machine is off but tap is On, because when I turn the unconnected tap on, the water comes gushing out ? Where does all that gushing out water go, how does it stop my water bill from being astronomical? Does the washing machine somehow create pressure to send the water back up to Thames Water when the washing machine is off or how does it work?)